Wind power plants comprising a plurality of wind turbines may be placed in rural areas where wind speed is high, but where the infrastructure of the electrical power system has deficiencies. In particular, a transmission system, the transformer and the connecting cable(s) may not be stable in all situations which may result in a weak power system or weak utility grid, having a particular high grid impedance. In particular, the strength (in particular the impedance) of the power system or the utility grid may change with time. In particular, the changes may be associated with connecting or disconnecting of generator, capacitor, transmission line, cables or transformers. In particular for a utility grid having a changing impedance the voltage delivered to a point of common coupling may not be kept in a predetermined range around a rated voltage.
There may be a need for a method and for an arrangement for controlling a wind turbine, which may in particular apply to a utility grid having changing impedance, and which may result in a more stable voltage at the point of common coupling.